fn main() {
    let immutable_box = Box::new(5i32);

    println!("immutable_box: {}", immutable_box);

    // 因为是不可变引用，所以不能改变值

    // *immutable_box = 10;

    // 所有权转移，不可变引用失效
    let mut mutable_box = immutable_box;

    println!("mutable_box: {}", mutable_box);

    *mutable_box = 20;

    println!("mutable_box: {}", mutable_box);
}